Polycarbonate is a useful engineering plastic for parts requiring clarity, toughness and heat resistance. However, polycarbonate also has deficiencies which include poor chemical and stress crack resistance, and poor processability. One method to improve these properties is to blend polycarbonate with another resin, a useful family of such resins being thermoplastic polyesters. Until now, however, such polycarbonate/polyester blends were able to increase processability only by decreasing impact strength.
Blends of polycarbonate with a copolyester derived from mixtures of isophthalic acid and terephthalic acid with 1,4 cyclohexanedimethanol are disclosed in U.S. Pat. Nos. 4,188,314 and 4,391,954. Such patents are directed to two component thermoplastic molding compositions containing an aromatic polycarbonate and a polyester resin derived from cyclohexanedimethanol and a mixture of iso- and terephthalic acids.
U.S. Pat. No. 5,010,146, is directed to a three component polyester/polycarbonate blend containing a poly(alkylene terephthalate), a polycarbonate and a polyester carbonate copolymer. The polyester carbonate copolymer is obtained by copolymerization of an alkylene terephthalate oligomer, preferably a polyethylene terephthalate oligomer, and a carbonate oligomer. The use of such oligomers is said to improve the crystallinity index of the compositions and thereby maintain such characteristics as chemical resistance without relying on the addition of other components which decrease impact resistance.
In contrast, the present invention relates to a blend comprised of a polycarbonate of 4,4'isopropylidenediphenol (bisphenol A polycarbonate), a poly(1,4-cyclohexylenedimethylene terephthalate) resin, and a copolyester which consists essentially of repeat units from mixtures of isophthalic acid, terephthalic acid and 1,4-cyclohexanedimethanol. The present inventors have unexpectedly determined that a copolyester may be incorporated into blends to increase the processability of the blends without decreasing impact strength if a polyester is added in conjunction with a copolyester. Thus, the blends of the present invention are able to achieve high impact strength without the loss of processability at low temperatures, compared to known polyester/polycarbonate blends.